全文获取类型
收费全文 | 13391篇 |
免费 | 1155篇 |
国内免费 | 11篇 |
专业分类
14557篇 |
出版年
2023年 | 48篇 |
2022年 | 126篇 |
2021年 | 227篇 |
2020年 | 140篇 |
2019年 | 184篇 |
2018年 | 205篇 |
2017年 | 171篇 |
2016年 | 327篇 |
2015年 | 561篇 |
2014年 | 584篇 |
2013年 | 789篇 |
2012年 | 949篇 |
2011年 | 971篇 |
2010年 | 630篇 |
2009年 | 554篇 |
2008年 | 826篇 |
2007年 | 865篇 |
2006年 | 773篇 |
2005年 | 745篇 |
2004年 | 765篇 |
2003年 | 709篇 |
2002年 | 652篇 |
2001年 | 142篇 |
2000年 | 90篇 |
1999年 | 143篇 |
1998年 | 165篇 |
1997年 | 122篇 |
1996年 | 114篇 |
1995年 | 102篇 |
1994年 | 93篇 |
1993年 | 107篇 |
1992年 | 99篇 |
1991年 | 67篇 |
1990年 | 78篇 |
1989年 | 59篇 |
1988年 | 73篇 |
1987年 | 73篇 |
1986年 | 56篇 |
1985年 | 78篇 |
1984年 | 79篇 |
1983年 | 78篇 |
1982年 | 104篇 |
1981年 | 94篇 |
1980年 | 88篇 |
1979年 | 54篇 |
1978年 | 52篇 |
1977年 | 53篇 |
1976年 | 53篇 |
1974年 | 49篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
No evidence for substantial aerobic methane emission by terrestrial plants: a 13C-labelling approach 总被引:2,自引:0,他引:2
Dueck TA de Visser R Poorter H Persijn S Gorissen A de Visser W Schapendonk A Verhagen J Snel J Harren FJ Ngai AK Verstappen F Bouwmeester H Voesenek LA van der Werf A 《The New phytologist》2007,175(1):29-35
* The results of a single publication stating that terrestrial plants emit methane has sparked a discussion in several scientific journals, but an independent test has not yet been performed. * Here it is shown, with the use of the stable isotope (13)C and a laser-based measuring technique, that there is no evidence for substantial aerobic methane emission by terrestrial plants, maximally 0.3% (0.4 ng g(-1) h(-1)) of the previously published values. * Data presented here indicate that the contribution of terrestrial plants to global methane emission is very small at best. * Therefore, a revision of carbon sequestration accounting practices based on the earlier reported contribution of methane from terrestrial vegetation is redundant. 相似文献
952.
Plant growth chambers are commonly used to minimize environmental variation but the light sources used vary considerably from natural light and from each other. Incandescent globes are often used to add more far-red light, with the aim of producing a more natural red to far-red ratio (R:FR), but also add to thermal load. High-intensity discharge lamps are often used to produce higher irradiances, more akin to natural light, but the thermal implications are rarely considered because air temperature is controlled. This paper examines the spectral properties and thermal implications of growth chamber light sources and takes a whole-plant physiology approach, by examining growth responses of a photoperiodic pea line ( Pisum sativum L. cv. Torsdag) in the same growth chamber type under different light sources – in essence using plants to study the controlled environments rather than vice-versa. High R:FR delayed flowering and inhibited internode extension in pea. However, the addition of far-red-rich incandescent globes in the proportions provided in the growth chambers (400–500 W) did little to reduce R:FR, did not induce earlier flowering and actually further inhibited internode length. Leaflet size and yield were significantly reduced. While air temperature was maintained at 20°C in all experiments, radiant temperature was significantly higher under high irradiance and/or with incandescent added, and soil temperatures were elevated. Growth responses under these lights were similar to the effect caused by elevating the air temperature. An alternative method of controlling R:FR, without thermal load implications, using light-emitting diodes is described. 相似文献
953.
Five 3-hydroxy-type homoisoflavonoids, 3,5-dihydroxy-7,8-dimethoxy-3-(3',4'-dimethoxybenzyl)-4-chromanone, 3,5-dihydroxy-7-methoxy-3-(3',4'-dimethoxybenzyl)-4-chromanone, 3,5-dihydroxy-7,8-dimethoxy-3-(3'-hydroxy-4'-methoxybenzyl)-4-chromanone, 3,5,6-trihydroxy-7-methoxy-3-(3'-hydroxy-4'-methoxybenzyl)-4-chromanone and 3,5,7-trihydroxy-3-(3'-hydroxy-4'methoxybenzyl)-4-chromanone in addition to the nortriterpenoid, 15-deoxoeucosterol, have been isolated from the dichloromethane extract of the bulbs of Pseudoprospero firmifolium, the sole representative of the tribe Pseudoprospereae of the subfamily Hyacinthoideae of the Hyacinthaceae. 相似文献
954.
Wei W Alexandersson E Golldack D Miller AJ Kjellbom PO Fricke W 《Plant & cell physiology》2007,48(8):1132-1147
The aim of the present study was to identify water channel(s) which are expressed specifically in the growth zone of grass leaves and may facilitate growth-associated water uptake into cells. Previously, a gene had been described (HvEmip) which encodes a membrane intrinsic protein (MIP) and which is particularly expressed in the base 1 cm of barley primary leaves. The functionality of the encoding protein was not known. In the present study on leaf 3 of barley (Hordeum vulgare L.), a clone was isolated, termed HvPIP1;6, which has 99% amino acid sequence identity to HvEmip and belongs to the family of plasma membrane intrinsic proteins (PIPs). Expression of HvPIP1;6 was highest in the elongation zone, where it accounted for >85% of expression of known barley PIP1s. Within the elongation zone, faster grower regions showed higher expression than slower growing regions. Expression of HvPIP1;6 was confined to the epidermis, with some expression in neighboring mesophyll cells. Expression of HvPIP1;6 in Xenopus laevis oocytes increased osmotic water permeability 4- to 6-fold. Water channel activity was inhibited by pre-incubation of oocytes with 50 microM HgCl(2) and increased following incubation with the phosphatase inhibitor okadaic acid or the plant hormone ABA. Plasma membrane preparations were analyzed by Western blots using an antibody that recognized PIP1s. Levels of PIP1s were highest in the elongation and adjacent non-elongation zone. The developmental expression profile of HvPIP2;1, the only known barley water channel belonging to the PIP2 subgroup, was opposite to that of HvPIP1;6. 相似文献
955.
Arabidopsis sucrose transporter AtSUC9. High-affinity transport activity, intragenic control of expression, and early flowering mutant phenotype 下载免费PDF全文
Sivitz AB Reinders A Johnson ME Krentz AD Grof CP Perroux JM Ward JM 《Plant physiology》2007,143(1):188-198
AtSUC9 (At5g06170), a sucrose (Suc) transporter from Arabidopsis (Arabidopsis thaliana) L. Heynh., was expressed in Xenopus (Xenopus laevis) oocytes, and transport activity was analyzed. Compared to all other Suc transporters, AtSUC9 had an ultrahigh affinity for Suc (K(0.5) = 0.066 +/- 0.025 mm). AtSUC9 showed low substrate specificity, similar to AtSUC2 (At1g22710), and transported a wide range of glucosides, including helicin, salicin, arbutin, maltose, fraxin, esculin, turanose, and alpha-methyl-d-glucose. The ability of AtSUC9 to transport 10 glucosides was compared directly with that of AtSUC2, HvSUT1 (from barley [Hordeum vulgare]), and ShSUT1 (from sugarcane [Saccharum hybrid]), and results indicate that type I and type II Suc transporters have different substrate specificities. AtSUC9 protein was localized to the plasma membrane by transient expression in onion (Allium cepa) epidermis. Using a whole-gene translational fusion to beta-glucuronidase, AtSUC9 expression was found in sink tissues throughout the shoots and in flowers. AtSUC9 expression in Arabidopsis was dependent on intragenic sequence, and this was found to also be true for AtSUC1 (At1g71880) but not AtSUC2. Plants containing mutations in Suc transporter gene AtSUC9 were found to have an early flowering phenotype under short-day conditions. The transport properties of AtSUC9 indicate that it is uniquely suited to provide cellular uptake of Suc at very low extracellular Suc concentrations. The mutant phenotype of atsuc9 alleles indicates that AtSUC9 activity leads to a delay in floral transition. 相似文献
956.
Fabbro T Davison AC Steinger T 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2007,115(7):933-944
Many quantitative genetic statistics are functions of variance components, for which a large number of replicates is needed
for precise estimates and reliable measures of uncertainty, on which sound interpretation depends. Moreover, in large experiments
the deaths of some individuals can occur, so methods for analysing such data need to be robust to missing values. We show
how confidence intervals for narrow-sense heritability can be calculated in a nested full-sib/half-sib breeding design (males
crossed with several females) in the presence of missing values. Simulations indicate that the method provides accurate results,
and that estimator uncertainty is lowest for sampling designs with many males relative to the number of females per male,
and with more females per male than progenies per female. Missing data generally had little influence on estimator accuracy,
thus suggesting that the overall number of observations should be increased even if this results in unbalanced data. We also
suggest the use of parametrically simulated data for prior investigation of the accuracy of planned experiments. Together
with the proposed confidence intervals an informed decision on the optimal sampling design is possible, which allows efficient
allocation of resources. 相似文献
957.
Marcelo R. de Carvalho Flávio A. Bockmann Dalton S. Amorim Carlos Roberto F. Brandão Mário de Vivo José L. de Figueiredo Heraldo A. Britski Mário C. C. de Pinna Naércio A. Menezes Fernando P. L. Marques Nelson Papavero Eliana M. Cancello Jorge V. Crisci John D. McEachran Robert C. Schelly John G. Lundberg Anthony C. Gill Ralf Britz Quentin D. Wheeler Melanie L. J. Stiassny Lynne R. Parenti Larry M. Page Ward C. Wheeler Julián Faivovich Richard P. Vari Lance Grande Chris J. Humphries Rob DeSalle Malte C. Ebach Gareth J. Nelson 《Evolutionary biology》2007,34(3-4):140-143
958.
Lightner DV Riggs A Corbin JS Ostrowski AC 《Diseases of aquatic organisms》2007,74(1):77-9; discussion 81-3
It is the opinion of the authors of the Comment on Do et al. (2006), that those authors incorrectly interpreted their test results, which are more likely the result of mislabeling of samples or within-laboratory contamination, and that the TSV isolates found in Penaeus vannamei in Korea in 2004 and 2005 did not originate from Hawaii as claimed by the authors, but from a country (or countries) in southeast Asia. Finally, we believe that the authors did not follow proper international guidelines, extend a professional courtesy to the supplier of the disputed shrimp sample, nor take a critical approach in interpreting their own data. It is unfortunate that the authors did not follow through with additional testing, or seek a second opinion from an independent laboratory, before implicating shrimp imported from Hawaii as the source of TSV in Korea. 相似文献
959.
Standifer NE Stacy S Kraig E Infante AJ 《Journal of immunology (Baltimore, Md. : 1950)》2007,178(6):3544-3550
Mice expressing the Torpedo acetylcholine receptor alpha-chain as a neo-self-Ag exhibit a reduced frequency of T cells responding to the immunodominant epitope Talpha146-162 indicating a degree of tolerance. We characterized tolerance induction in these animals by analyzing the residual Talpha146-162-responsive T cell population and comparing it to that of nontransgenic littermates. Using CD4(high) sorting, we isolated the vast majority of Ag-reactive T cells from both strains of mice. Quantitative studies of the CD4(high) populations in transgenic mice following immunization with Talpha146-162 revealed a diminished expansion of cells expressing the canonical TCRBV6 but not other TCRBV gene segments when compared with nontransgenic littermates. In addition, CD4(high) cells from transgenic mice were functionally hyporesponsive to Talpha146-162 in terms of proliferation and cytokine secretion regardless of TCRBV gene segment use. TCR sequence analysis of transgenic Vbeta6(+)CD4(high) cells revealed a reduced frequency of cells expressing a conserved motif within the TCRbeta CDR3. Thus, the canonical Talpha146-162 responsive, Vbeta6(+) population demonstrates both quantitative and qualitative deficits that correlate with an altered TCR repertoire whereas the non-Vbeta6 population in transgenic mice exhibits only a reduction in peptide responsiveness, a qualitative defect. These data demonstrate that discrete autoreactive T cell populations with identical peptide/MHC specificity in Torpedo acetylcholine receptor-alpha-transgenic animals bear distinct tolerance imprints. 相似文献
960.
Salmonella induces a switched antibody response without germinal centers that impedes the extracellular spread of infection 总被引:1,自引:0,他引:1
Cunningham AF Gaspal F Serre K Mohr E Henderson IR Scott-Tucker A Kenny SM Khan M Toellner KM Lane PJ MacLennan IC 《Journal of immunology (Baltimore, Md. : 1950)》2007,178(10):6200-6207
T-dependent Ab responses are characterized by parallel extrafollicular plasmablast growth and germinal center (GC) formation. This study identifies that, in mice, the Ab response against Salmonella is novel in its kinetics and its regulation. It demonstrates that viable, attenuated Salmonella induce a massive early T-dependent extrafollicular response, whereas GC formation is delayed until 1 mo after infection. The extrafollicular Ab response with switching to IgG2c, the IgG2a equivalent in C57BL/6 mice, is well established by day 3 and persists through 5 wk. Switching is strongly T dependent, and the outer membrane proteins are shown to be major targets of the early switched IgG2c response, whereas flagellin and LPS are not. GC responses are associated with affinity maturation of IgG2c, and their induction is associated with bacterial burden because GC could be induced earlier by treating with antibiotics. Clearance of these bacteria is not a consequence of high-affinity Ab production, for clearance occurs equally in CD154-deficient mice, which do not develop GC, and wild-type mice. Nevertheless, transferred low- and high-affinity IgG2c and less efficiently IgM were shown to impede Salmonella colonization of splenic macrophages. Furthermore, Ab induced during the infection markedly reduces bacteremia. Thus, although Ab does not prevent the progress of established splenic infection, it can prevent primary infection and impedes secondary hemogenous spread of the disease. These results may explain why attenuated Salmonella-induced B cell responses are protective in secondary, but not primary infections. 相似文献